PIREP: Five years ago when I needed to replace my 121.5 old unit, I did a lot of research on ELTs. The Pointer Avionics SkyHunter 406 ELT was definitely the best choice out there at the time with its built-in GPS. Also, I learned that a higher than acceptable percentage of the time ELTs do not go off because of the attitude of the impact, airplane ELTs only go off in the forward direction. People have died after surviving the crash but found too long to be saved (Steve Fossett??). I learned that helicopter ELTs will activate in any inertial direction of impact. Needless to say, if you crash, you want your ELT to work, every time; "usually" is not good enough, and that is the model I bought. This is worth thinking about whatever brand you buy.
Anyway, I did buy the SkyHunter and all has been fine until now. It is five years later and the battery is now due for replacement. I found out that Pointer Avionics is either out of business or not making ELTs anymore, in either case, batteries are no longer available. Luckily (and the reason I am posting this), I found a thread on another site that MERL in Meriden, Connecticut (203.237.8811) is able to "overhaul" the battery and certify it airworthy for another five years.
They charge $300 for the battery and $135 for testing and recertifying the ELT. They say they typically turn it around in a couple days. Rob also said they are trying to work out an arrangement with the guy who owned the now-defunct Pointer Avionics so they can do the batteries as a PMA, hopefully, that will happen before five years runs down to my next replacement.
Also, they mentioned if you have a spare ELT sitting around the hangar, they may buy it from you for parts.